Each line to the telephone sets required six wires from the key system:
One pair two wires carried the talk circuit,
A second pair carried control information, known as A-Leads, for that line,
A third pair carried current to a lamp for the specific line key position on the telephone set.
Cross-connect wire jumpers, consisting of three twisted pairs the six wires referenced above were installed between these blocks and the larger distribution connecting blocks within the KSU for each line provided to the telephone set.
Very large 1A2 installations had multiple wiring closets fed by branch cables extended from the central closet where the KSU was located. An example of this type of installation would be a multi-story building. The KSU and incoming lines might be in the basement, while each floor had a branch wiring closet to which the telephones for that floor were connected.
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